99 GC Rear end makes noise when put into gear

[gatordp]

In the morning when I first shift into reverse, there is a low pitched wine that sounds like it is coming from the rear dif. It lasts about 10 seconds then stops and doesn't come back. The noise happens while sitting still and continues while backing up. It will also make the same noise if I shift into drive. After it stops, it does not come back for the rest of the day. It also does not happen every day. Only about 1 in five. I have about 120,000 miles and replaced the dif fluid at about 70k. Any ideas.

[Bob D.]

Does it do it in reverse while not moving?
If so it could be transmission regulator valve resonance.

[gatordp]

Yes it will make the noise while sitting still while it is in gear and my foot on the brake. It will continue as I move. It stops after a few seconds whether I am moving or sitting still. It sounds like the noise is coming from further back than the transmission, but if the drive shaft is not moving, it doesn't make sense that it is in the rear end. I just checked my transmission fluid, and it is a little above the max and there are a lot of small bubbles in it. It looked nice and clean.

[Bob D.]

Originally Posted by gatordp

...there are a lot of small bubbles in it.

That may be what's causing the noise - air in the fluid.